Some proposals have been made regarding a thermally color-developing reversibly thermochromic material (cf. FIG. 4) which shows a colored state upon heating from a discolored state and discolors upon cooling from the colored state within the life environmental temperature range (e.g., U.S. Pat. No. 5,919,404).
As shown in FIG. 4, this conventional thermally color-developing reversibly thermochromic material has a characteristic of having large variable color density within the coloring temperature range, showing a color-changing behavior in which the color density is drastically reduced at a high temperature side (Tx) bordering the complete coloration temperature (T2) and drastically increased at a low temperature side reaching a decoloration initiation temperature (T3).